A dielectric filter in which a resonator is obtained by forming an inner conductor and an outer conductor at a dielectric block is widely used (see, for example, Patent Document 1).
FIGS. 1(A) and 1(B) are perspective views of a dielectric filter in the prior art disposed on a mount board as viewed from an upper surface. A left front surface of a dielectric filter in the drawing is hereinafter referred to as a front surface, and a right front surface of the dielectric filter in the drawing is hereinafter referred to as a right side surface. FIG. 1(A) is a diagram describing a dielectric filter disposed on a mount board 201. FIG. 1(B) is a diagram describing a dielectric filter disposed on a mount board 301.
A dielectric filter 101 includes a dielectric block 102. The dielectric block 102 is a rectangular parallelepiped, and includes three resonators. On an outer surface excluding a front surface that is an open surface of the dielectric block 102, an outer conductor 103 and input/output electrodes 104A and 104B are disposed. Inner conductor formation holes passing from the front surface to the back surface of the dielectric block 102 are disposed. Inner conductors 105A, 105B, 105C are individually disposed on the inner surfaces of the inner conductor formation holes. Each of the inner conductors 105A to 105C has one end on the front surface of the dielectric block 102 as an open end and the other end connected to the outer conductor 103 on the back surface of the dielectric block 102. In order to obtain strong external coupling between an input (output) resonator formed by the inner conductor 105A and the input/output electrode 104A and between an output (input) resonator formed by the inner conductor 105C and the input/output electrode 104B, the input/output electrodes 104A and 104B are individually disposed near the open ends of these resonators. More specifically, the input/output electrode 104A is formed from a corner formed by the front surface and the left side surface of the dielectric block 102 on the undersurface of the dielectric block 102 to the left side surface of the dielectric block 102, and the input/output electrode 104B is formed from a corner formed by the front surface and the right side surface of the dielectric block 102 on the undersurface of the dielectric block 102 to the right side surface of the dielectric block 102.
When the dielectric filter 101 is disposed on a mount board 201, the outer conductor 103 is connected to a ground electrode 202 of the mount board 201 and the input/output electrodes 104A and 104B are individually connected to signal lines 203A and 203B of the mount board 201 on the undersurface of the dielectric block 102.
The mount board 201 illustrated in FIG. 1(A) includes coplanar signal lines 203A and 203B. The signal lines 203A and 203B extend so that they are parallel to each other. On the mount board 201, the dielectric filter 101 is disposed so that the front surface thereof is perpendicular to a direction in which the signal lines 203A and 203B extend. The signal lines 203A and 203B are connected to the input/output electrodes 104A and 104B from the front surfaces of the input/output electrodes 104A and 104B, respectively.
The mount board 301 illustrated in FIG. 1(B) includes coplanar signal lines 303A and 303B, and a ground electrode 302. The signal lines 303A and 303B are disposed on the same line so that the leading ends thereof are apart by a fixed distance. On the mount board 301, the dielectric filter 101 is disposed so that the right side surface thereof and the left side surface thereof are perpendicular to a direction in which the signal lines 303A and 303B extend. The signal lines 303A and 303B are connected to the input/output electrodes 104A and 104B from the side surfaces of the input/output electrodes 104A and 104B, respectively.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 200-174502
In the above-described dielectric filter, since the ground electrode of the mount board is at a ground potential, a ground current flows through the outer conductor on the side surfaces and the upper surface of the dielectric block. Accordingly, the potential of the outer conductor on the dielectric block is substantially the same as the ground potential.
In reality, however, since a part of the outer conductor having a length corresponding to the wavelength of a target high-frequency signal for the dielectric filter is apart from the ground electrode, the potential of the part of the outer conductor is apart from the ground potential at a high frequency. In the above-described dielectric filter, since the input/output electrode is disposed on the undersurface and the side surface of the dielectric block on the side of the front surface (open surface) of the dielectric block, the potential of a part of the outer conductor on the upper sides of the input/output electrodes disposed on the sides of the dielectric block is easily apart from the ground potential at a high frequency. Accordingly, the potential of the upper surface of the dielectric block is easily apart from the ground potential at a high frequency.
In order to prevent the potential of the outer conductor on the upper surface from being apart from the ground potential at a high frequency, a cover is sometimes attached to the open surface so as to make a short circuit between the outer conductor on the upper surface and the ground electrode. In this case, the number of steps and a footprint are increased due to attachment of a cover to the open surface.